I agree with how Beast's story was revamped, however there are some differences between Beast and Karlach.
First, Beast isn't nearly as popular as Karlach is, and if I recall correctly there wasn't really much outrage surrounding his questline, rather just some pointing out of how it felt lackluster. Karlach meanwhile has a massive following and is routinely cited as the biggest problem that people have with Act 3 and the game in general. You could better compare it with Fane than Beast, whose endings were legitimately bad and were also updated with the Definitive Edition.
Second, Beast's storyline wasn't really bad, it was just kinda... boring. It really only needed to be expanded, it's problems weren't really that glaring. Karlach by contrast not only contradicts in game lore and the solutions that the game provides us, but also potentially ruins other endings, like retiring with Wyll or wanting to self-sacrifice without sentencing Karlach to a miserable death.
Third, Larian is a different place than it was in 2017. BG3 is hugely successful, and Larian has far more in the way of resources and the ability to be agile about incorporating player feedback. We aren't even 2 months since release and we've gotten: -The ability to change appearance midgame -Better multiplayer system -A new Karlach epilogue scene -A (somewhat dubious) inventory upgrade -Countless bugfixes and minor story adjustments
They are quite clearly invested in incorporating the player feedback they've received, and just deciding they want to railroad Karlach into having a bad ending despite what most people are telling them that's not what they want would be an honestly baffling decision. I'm not saying it isn't a possibility that Larian will just add some dialogue about accepting Karlach's fate and moving on, but honestly I feel like if they were going to do that they would have done it by now. There's also no reason to think that even if they did decide to go that route, that the community wouldn't loudly complain about it until it's fixed.
